03. Test Doubles

Test Doubles

Types of Test Doubles

A **test double **is any kind of object that acts as a stand-in for an expected dependency in order to facilitate testing.

Dummy: A class that doesn't actually do anything, but fills a required parameter of some method.

Stub: A simple class that always returns a hard-coded value.

Fake: A test double that allows you to customize the response to imitate actual functionality.

Spy: A test double that can keep track of which methods were called. Sometimes they can also track what parameters were used or how often the methods were called.

Mock: A generic term for test doubles, or more specifically, a test double that can prepare specific responses for different kinds of input.

Examples

public class DummySalesService implements SalesService {
   @Override
   public String fizzBuzz(int i) {
   }
}

public class StubSalesService implements SalesService {
   @Override
   public String fizzBuzz(int i) {
       return "Buzz"; //always returns Buzz
   }
}

public class FakeSalesService implements SalesService {
   private String returnValue;
   public FakeSalesService(String returnValue) {
       this.returnValue = returnValue;
   }
   @Override
   public String fizzBuzz(int i) {
       return returnValue;
   }
}

public class SpySalesService implements SalesService {
   //tracks number of calls and the parameters used
   private int numFizzBuzzCalled; 
   private List<Integer> parameters = new ArrayList<>(); 
   @Override
   public String fizzBuzz(int i) {
       numFizzBuzzCalled++;
       parameters.add(i);
       return null;
   }
   public int getNumFizzBuzzCalled() { return numFizzBuzzCalled; }
   public List<Integer> getParameters() { return parameters; }
}

public class MockSalesService implements SalesService{
   //stores responses for inputs
   private Map<Integer, String> returnValues = new HashMap<>();
   @Override
   public String fizzBuzz(int i) {
       return returnValues.getOrDefault(i, null);
   }
   //allows users to customize return values programmatically
   public void setReturnForInput(int i, String val) {
       returnValues.put(i, val);
   }
}